tritium_pie

since Jan 8th, I've noticed a drop in my MPG. I've been tracking this on an Excel spreadsheet-- with graphs-- since I bought my car, and I have noticed a trend of gradually increasing fuel efficiency-- which I assume is related to the engine breaking in well, and keeping it well oiled. it also made me smile.

my driving habits haven't changed (it's my weekend toy), and in fact after first noticing this, I tried driving much more efficiently, using cruise control on the hwy at 65mph, etc. this hasn't helped.

for example, the trendline shows my fuel efficiency running at around 23.5 MPG when I first got the car in June 03 and trending upwards to where it reached 25 MPG by June 05, and was at > 25.5 MPG last December, but after Jan of this year I'm seeing an average of 23 MPG. Not gigantic, but it is a 10% drop, and it does go against the trend. granted, I know my car wasn't going to keep getting more and more efficient forever... but I would've expected it to plateau... not suddenly drop 10%.

I'm at 29K miles, so maybe my plugs are needing to get changed? I know I'm due for the 30K mile check-up that the manual calls for. the oil-- as always-- is topped off. and I always buy Chevron Supreme.

any thoughts?

S2KinVA

Could be a million things really, How often if ever do you change/clean your air filter? New plugs or regap of the current ones may help...

If you do the plugs yourself, look to see if there is a whitish buildup on the tips, if not and you see a rough brownish tip then your car is running rich. That could be a possible reason for the lower mileage. You may want to check out the UTH section for more info as well.

Also, winter gas doesnt seem to burn as well as the summer blend...

WPS

gas type
tire pressure?

xviper

Aside from everything that has already been posted, also replace the PCV valve (cheap and easy), use some concentrated injector cleaner now and then do the maintenance dose at each oil change interval. You fuel sock may have also been overworked with dirty gas, but this is impossible to tell without taking the thing out and looking at it or checking the fuel pressure. If you go to the effort the "check" the fuel sock, may as well just get a new one and change it.
Is your timing chain tensioner making more noise than usual? Your valve timing could be off just a bit due to a tensioner in need or replacement.
If you don't customarily change fluids and they get really dirty, they will offer more resistance. I'm talking about the rear diff mainly but also will include the tranny. As the car gets older, things like wheel bearings don't spin as easily. This doesn't mean you need to go out and change them, but it's just another one of those "little" things that, when all added up, can contribute to an overall increase in rolling resistance.

robin2race

check tire pressure~!

tritium_pie

thanks for all the responses.

regarding my air filter, that was changed on Jan 1st @ 26742 miles, so it can't be that.

thanks everyone for the suggestions-- I'll have to check them off one by one.

hirev

i live in the bay area and experience elevations and differing driving conditions plus plenty of highway 280 time...i get almost exactly the mpg you do on an average basis...i have noticed....

in winter the mileage goes down due to crappy gas, we get it in the winter time, it is an epa additive or something like that so it is expected for mileage to drop due to that...if you punch it at least a couple of gears worth getting onto the freeway you can drop 1 mpg or so, drive it without going over 6000 rpm and you get superb mileage..and that is really not part of enjoying the car, so i don't do it, but every once in a while i can improve my mpg by just not going into vetc durring a tank of gas, or less than once a day, etc...

that would be my answer, i think your mileage is par and normal and excellent considering you are driving a amazing car with an even more amazing engine,the pickup of a v6 or even small v8 with the mileage of a nb miata.

Jsmply

I agree with the above statements. It is not a huge decrease, I would not worry about it to much. Do the maintance as you see fit and just keep an eye on it. If it dropped 25% then I would wonder, but like everyone else has said the amount of things that could have added a tad of resistance is huge. All in all, you sound like your in the norm to me.

I reset my trip meter every time I get a tank of gas. I have no idea why I still do this as I don't even calculate it anymore when filling up because its always between 20mpg and 24mpg depending on how hard I have driven the tank. You still got to cover the same ground either way, so as long as it is not a big drop that could be a sign of a major problem, I'd just see if regular maintenance over time changes things and not go out of your way trying to get that extra 1-2mpg back. You'll probably end up putting quite a few miles on your car that you didn't enjoy just because you were trying to get your graph up to par.

Either way, good luck and good job at keeping a close eye on your car!

TR-S2K

When did we starting to use Ethanol in place of MTBE as oxygenate in our fuel ? Is it starting Jan of this year ? Gasoline with 10%Ethanol does not have as much energy as gasoline with MTBE, so that cars using it get lower gas mileage. Even Ethanol website said that the average lost in gas mileage is 2% for gasoline with 10% Ethanol.

S2k Dude

I totally agree with the above statement. I believe you will see a 2-3 MPG drop in fuel economy with more and more states adopting the mandatory 10% ethanol fuel mixture that is a result of our government thinking they're doing all of us a favor.

I've read that ethanol actually has about 30% less energy density compared to gasoline but when you only add 10% ethanol to the mixture that equates to about a 3%-4% decrease in MPG which, for an S2000, about 1-2 less MPG or about 20 miles less per tank which is exactly what I've been seeing consistently ever since the Atlanta Metro area started the mandatory 10% ethanol at the local pumps.

So my recommendation is to do some research and see if the gasoline in your area has the 10% ethanol mix.